Pyrolysis process for producing fuel gas

ABSTRACT

Solid waste resource recovery in space is effected by pyrolysis processing, to produce light gases as the main products (CH 4 , H 2 , CO 2 , CO, H 2 O, NH 3 ) and a reactive carbon-rich char as the main byproduct. Significant amounts of liquid products are formed under less severe pyrolysis conditions, and are cracked almost completely to gases as the temperature is raised. A primary pyrolysis model for the composite mixture is based on an existing model for whole biomass materials, and an artificial neural network models the changes in gas composition with the severity of pyrolysis conditions.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of application Ser. No. 09/902,425,filed Jul. 10, 2001, now U.S. Pat. No. 7,169,197 the entirespecification of which is incorporated hereinto by reference thereto.

STATEMENT REGARDING GOVERNMENT INTEREST

The United States Government has rights in this invention under NASAContracts NAS2-99001 and NAS2-00007.

BACKGROUND OF THE INVENTION

The NASA objective of expanding the human experience into the farreaches of space will require the development of regenerable lifesupport systems. A key element of these systems is a means for solidwaste resource recovery. The objective of this invention is todemonstrate the feasibility of pyrolysis processing as a method for theconversion of solid waste materials in a Controlled Ecological LifeSupport System (CELSS). A pyrolysis process will be useful to NASA in atleast four respects: 1) it can be used as a pretreatment for acombustion process; 2) it can be used as a more efficient means ofutilizing oxygen and recycling carbon and nitrogen; 3) it can be used tosupply fuel gases to fuel cells for power generation; and 4) it can beused as the basis for the production of chemicals and materials inspace.

A key element of a CELSS is a means for solid waste resource recovery.Solid wastes will include inedible plant biomass (IPB), paper, plastic,cardboard, waste water concentrates, urine concentrates, feces, etc. Itwould be desirable to recover usable constituents such as CO₂, H₂O,hydrogen, nitrogen, nitrogen compounds, and solid inorganics. Anyunusable byproducts should be chemically and biologically stable andrequire minimal amounts of storage volume.

Many different processes have been considered for dealing with thesewastes: incineration, aerobic and anaerobic biodigestion, wet oxidation,supercritical water oxidation, steam reforming, electrochemicaloxidation and catalytic oxidation. However, some of these approacheshave disadvantages which have prevented their adoption. For example,incineration utilizes a valuable resource, oxygen, and producesundesirable byproducts such as oxides of sulfur and nitrogen.Incineration also will immediately convert all of the waste carbon toCO₂, which will require storing excess CO₂.

“Pyrolysis,” in the context of this application, is defined as thermaldecomposition in an oxygen-free environment. Primary pyrolysis reactionsare those which occur in the initial stages of thermal decomposition,while secondary pyrolysis reactions are those which occur upon furtherheat treatment. A pyrolysis based process has several advantages whencompared to other possible approaches for solid waste resourcerecovery: 1) it can be used for all types of solid products and can bemore easily adapted to changes in feedstock composition than alternativeapproaches; 2) the technology is relatively simple and can be madecompact and lightweight and thus is amenable to spacecraft operations;3) it can be conducted as a batch, low pressure process, with minimalrequirements for feedstock preprocessing; 4) it can produce severalusable products from solid waste streams (e.g., CO₂, CO, H₂O, H₂, NH₃,CH₄, etc.); 5) the technology can be designed to produce minimal amountsof unusable byproducts; 6) it can produce potentially valuable chemicalsand chemical feedstocks (e.g., nitrogen rich compounds for fertilizers,monomers, hydrocarbons); and 7) pyrolysis will significantly reduce thestorage volume of the waste materials while important elements such ascarbon and nitrogen can be efficiently stored in the form of pyrolysischar and later recovered by gasification or incineration when needed. Inaddition to being used as the primary waste treatment method, pyrolysiscan also be used as a pretreatment for more conventional techniques,such as incineration or gasification.

The primary disadvantages of pyrolysis processing are: 1) the productstream is more complex than for many of the alternative treatments; and2) the product gases cannot be vented directly in the cabin withoutfurther treatment because of the high CO concentrations. The formerissue is a feature of pyrolysis processing (and also a potentialbenefit, as discussed above). The latter issue can be addressed byutilization of a water gas shift reactor or by introducing the productgases into an incinerator or high temperature fuel cell.

SUMMARY OF THE INVENTION

It is a primary object of the present invention to provide a novelprocess and system by which non-gaseous hydrocarbonaceous materials, andparticularly mixed solid waste materials, can be converted to usablegases, as the main products, and to a reactive carbon-rich char as themain byproduct.

More specific objects of the invention are, as noted above, to providesuch a process which is feasible for use in a controlled ecological lifesupport system, and to provide a system in which the process isimplemented.

It has now been found that certain of the foregoing and related objectsare attained by the provision of a process for producing fuel gases fromat least one non-gaseous hydrocarbonaceous material, using a two-stagereaction apparatus, comprising the following steps, carried outcyclically:

(a) introducing a non-gaseous hydrocarbonaceous material into apyrolysis chamber, comprising a first stage of the apparatus, andpyrolyzing the hydrocarbonaceous material therein, usually at atemperature of about 600° to 900° C. (but necessarily substantiallylower that the second stage temperature), so as to produce a primaryfuel gas mixture, a pyrolysis liquid (condensed hydrocarbons), and afirst carbonaceous residue;

(b) introducing the primary fuel gas mixture and pyrolysis liquid into asecond chamber, comprising a second stage of the apparatus andcontaining a silica gel-based catalyst, and heating the liquid therein,in a substantially non-oxidizing atmosphere, to a temperature of about900° to 1100° C., so as to produce additional fuel gases and additionalsolid carbonaceous residue, without substantially altering thecomposition of the primary fuel gas mixture;

(c) withdrawing the primary fuel gas mixture and the additional fuel gasfrom the second chamber; and

(d) introducing air, oxygen, carbon dioxide or steam into each of thechambers to effect reaction with, and at least partial removal of, thecarbonaceous residue therein.

The primary fuel gas mixture produced by pyrolysis will usually consistprimarily of carbon monoxide, methane, and hydrogen. Steam, carbondioxide, or a mixture thereof will preferably be introduced into eachchamber so as to produce further quantities of fuel gas, andadvantageously to effect regeneration of the catalyst in the secondchamber as well.

In most embodiments, the steps of the process will be controlled byelectronic data processing means, programmed to monitor the formation ofat least one gas phase intermediate product, and preferably to monitorat least three such products. The gaseous product or products monitoredwill normally be selected from the group consisting of hydrogen,methane, carbon monoxide, carbon dioxide, water, and oxygen, and theymay constitute intermediate or final products. The data processing meanswill generally be programmed to determine the concentrations of the gasphase constituents, and to implement an artificial neural network modelbased thereupon, the concentrations determined being utilized as inputdata to the network. The neural network model will normally beconstructed to produce a fuel gas product of selected composition, froma specified hydrocarbonaceous material, by controlling the operatingparameters for the first and second stages of the apparatus.

Other objects of the invention are attained by the provision of a powergeneration system comprising a gas-fueled power generator; two-stagereaction apparatus for producing a fuel gas product from ahydrocarbonaceous material, operatively connected to supply fuel gas tothe power generator; and means for controlling the flow of fuel gas fromthe reaction apparatus to the generator, the reaction apparatus beingconstructed for effecting the cyclical fuel gas-producing process hereindescribed. Normally, the system will include electronic data processingmeans for controlling the steps of the process, programmed to monitorgas phase products in the manner set forth.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 comprises curves of TG-FTIR pyrolysis data for major products forNIST Wheat Straw at 10° C./min; a) time-temperature history, balancecurve from TGA (thermogravimetric analyzer) and sum of gases from FT-IR;b-f differential and cumulative evolution curves for major volatileproducts;

FIG. 2 comprises curves of TG-FT-IR pyrolysis data for major productsfor composite mixture at 10° C./min; a) time-temperature history,balance curve from TGA and sum of gases from FT-IR; b-f) differentialand cumulative evolution curves for major volatile products;

FIG. 3 comprises curves presenting comparisons of FG-DVC modelpredictions (solid lines) and TG-FT-IR pyrolysis data (symbols connectedby lines) (30° C./min) for composite mixture using modified wheat strawinput parameters;

FIG. 4 is a plot comparing measured and ANN model predicted CO yieldsfor post-pyrolyzer experiments with the composite mixture under slow andfast flow conditions;

FIG. 5 is a plot comparing ANN model predictions (lines) and measuredyields (symbols) of major gas products for post-pyrolyzer experimentswith the composite mixture under the fast flow conditions; and

FIG. 6 is a diagrammatic representation of a power generation systemembodying the present invention.

DETAILED DESCRIPTION OF SPECIFIC AND PREFERRED EMBODIMENTS

A model waste feedstock was used, consisting of 10 wt. % polyethylene,15% urea, 25% cellulose, 25% wheat straw, 10% of a coconut oil soap,i.e., sodium methyl cocyl taurate (Gerepon TC-42), and 5% methionine.The materials that were obtained and the elemental compositions of eachare given in Table 1. The samples in Table 1 were obtained and subjectedto thermogravimetric analysis with FT-IR analysis of evolved gases(TG-FTIR) at 10° C./min and 30° C./min. Details of the TG-FTIR methodcan be found in the prior art. The apparatus consists of a samplesuspended from a balance in a gas stream within a furnace. As the sampleis heated, the evolving volatile products are carried out of the furnacedirectly into a 5 cm diameter gas cell (heated to 150° C.) for analysisby FT-IR.

TABLE 1 Elemental Analysis of Individual and Composite Samples (wt. %)Sample Basis Moisture Ash C H O S N Polyethylene^(a) DAF 85.7 14.3 0.00.0 0.0 (Aldrich) Cellulose^(b) AR 5.0 (Avicel PH-102) D <0.05 44.0 6.249.8 ~0.0 ~0.0 DAF 44.0 6.2 49.8 ~0.0 ~0.0 Wheat Straw^(b) AR 7.9 (NIST)D 9.0 43.7 5.6 40.9 0.2 0.6 DAF 48.0 6.2 44.9 0.2 0.7 Urea^(a) (Aldrich)DAF 20.0 6.7 26.6 0.0 46.7 Gerepon^(c) D 7.6 55.9 10.6 10.6 10.6 4.7TC-42 (Rhône-Poulenc) DAF 60.5 11.5 11.5 11.5 5.0 Methionine^(a) DAF40.3 7.4 21.4 21.5 9.4 (Aldrich) Composite D 3.8 DAF 48.7 8.2 31.0 3.48.7 Notes: AR = As-received; D = Dry; DAF = Dry, Ash Free ^(a)=determined from chemical formula ^(b)= determined by HuffmanLaboratories (Golden, CO) ^(c)= estimated from approximate chemicalformula

In the standard analysis procedure, a ˜35 mg sample is taken on a 30°C./min temperature excursion in helium, first to 150° C. to dry, then to900° C. for pyrolysis. After cooling, a small flow of O₂ is added to thefurnace and the temperature is ramped to 700° C. (or higher) foroxidation in order to measure the inorganic residue. The TG-FTIR systemcan also be operated with a post pyrolysis attachment to examinesecondary pyrolysis of the volatile species (see below).

During these excursions, infrared spectra are obtained approximatelyonce every forty-one seconds. The spectra show absorption bands forinfrared active gases, such as CO, CO₂, CH₄, H₂O, C₂H₄, HCl, NH₃, andHCN. The spectra above 300° C. also show aliphatic, aromatic, hydroxyl,carbonyl and ether bands from tar (heavy liquid products). The evolutionrates of gases derived from the IR absorbance spectra are obtained by aquantitative analysis program. The aliphatic region is used for the tarevolution peak. Quantitative analysis of tar is performed with the aidof the weight-loss detain the primary pyrolysis experiments.

The TG-FTIR method provides a detailed characterization of the gas andliquid compositions and kinetic evolution rates from pyrolysis ofmaterials under a standard condition. While the heating rates are slower(3-100° C./min) than what is used in many practical processes, it is auseful way of benchmarking materials and was used in this study forcharacterizing both the primary and secondary pyrolysis behavior of themodel waste samples and the individual components.

Measurements of the thermodynamics of the pyrolysis process were madeusing differential scanning calorimetry (DSC). The DSC experiments weredone by heating at 10, 30 and 60° C./min. These heating rates were thesame or similar to the heating rates used in the TG-FTIR experiments, soa direct comparison could be made. A TA Instruments 2910 DSC system,with a maximum operating temperature of 600° C., was employed in the DSCwork. The sample cell was operated under a nitrogen flow rate of 100cm³/min in order to keep the cell free of oxygen during themeasurements. In preliminary work, this was noted to be important. Smallamounts of oxygen, participating in a combustion reaction, cansignificantly influence the thermal characteristics of the process.

Aluminum sample pans were used for the DSC experiments in a partiallysealed mode. This was done by pushing down the top sample pan covergently onto the bottom pan containing the sample. Following this, threesmall pinholes were poked into the sample pan to allow a limited amountof mass loss from the pan. This configuration gives results which areconsistent with pyrolysis in a confined system with a slow rate of massbleed out of the system, and are regarded to be reasonablyrepresentative of a pyrolysis processing system. Typically, about 10 mgof sample was used in an experiment. In many cases, particularly withcharring samples, the initial DSC run was followed by a cooling of thesample to room temperature, followed by a retrace of the originalheating profile. This procedure provides a background trace attributableto the heat capacity of the char residue. In cases involving formationof a char residue, the mass loss of the sample during the first heatingwas also established. These values were compared with the TG-FTIRresults, to verify whether the pyrolysis was occurring in a consistentmanner, or in a different manner due to the increased mass transportresistance in the DSC pans.

TG-FTIR Results for Primary Pyrolysis

Examples of some representative data are shown in FIGS. 1 and 2 for thewheat straw sample and for the composite mixture, respectively. Theseresults are for runs done at 10° C./min. For each of the samples, thedata are plotted in a six panel format. In each case, the panels include(a) temperature, sum of gases (top curve), and weight loss (bottomcurve) and (b-f) the differential and integral yields of tar, CH₄, H2O,CO₂, and CO as major pyrolysis products. In most cases, the minorpyrolysis products which are routinely quantified and plotted includeSO₂, C₂H₄, CS₂, NH₃, COS, and olefins. In many cases, the amounts ofthese latter product are barely above the noise level. Hydrogen is notreported since the gas is not IR active. However, only small amounts ofhydrogen are formed in primary pyrolysis experiments (<1 wt. %). It canbe an important product from secondary pyrolysis experiments and forthese experiments, the FT-IR measurements were supplemented by gaschromatography (GC) (see below).

Wheat straw—As expected, wheat straw produces oxygenated gases inaddition to tar. However, the wheat straw produces about 20-25 wt. %char (fixed carbon plus ash) on an as-received basis. The formation offixed carbon from whole biomass is known to result primarily from thearomatic lignin component of the plant, which typically comprises 20-25%by weight, with the remainder being primarily cellulose andhemicellulose. Previous work has shown that the weight loss frompyrolysis for whole biomass samples can be understood as a linearsuperposition of these three main components to a first approximation.However, one can not predict the yields of individual gas species usingthis approach, probably due to the catalytic effects of the traceminerals present in whole biomass.

Composite mixture—The results for TG-FTIR runs with the compositemixture are shown in FIG. 2. In terms of product distribution (char,tar, gas), the results are much more similar to the wheat straw samplethan the cellulose, polyethylene, Gerepon, methionine, or urea samples.This result makes sense in that the wheat straw is also amulti-component mixture which consists of cellulose, hemi-cellulose, andlignin, while the composite mixture is made up of 25% cellulose and 25%wheat straw as the largest components. The wheat straw sample also hasan elemental composition which is relatively close to that of thecomposite mixture (see Table 1). Therefore, one might expect similarpyrolysis behavior.

DSC Experiments

The general conclusion which can be drawn from these measurements isthat the composite mixture pyrolysis is only mildly endothermic (oforder 100 J/g), under conditions in which a significant amount of massloss is permitted to occur during pyrolysis. Confining pyrolysis morecompletely might be expected to drive the process in an even moreexothermic direction, as it does in the case of pure cellulose. In anyevent, it may be noted that, in comparison to this relatively modestenthalpy of pyrolysis, the sensible enthalpy for heating the sample isquite a bit larger. For example, using a “typical” average heat capacityfor cellulose of 2 J/g-K to represent the composite mixture, it may beseen that heating from room temperature to 600° C. will itself require1150 J/g of sample. Additionally, the heat required to evaporate anyresidual moisture content could also far outweigh this small pyrolysisthermal demand. Thus, it may be concluded that the heat of pyrolysiswill not be of significant design concern unless conditions far removedfrom these are to be explored. Most of the heat input required will beto overcome heat losses from the reactor.

CHAR Characterization

Reactivity measurements were made using indices known as T_(critical)and T_(late). These measure the temperature at which a char heated at30° C./min in air achieves a reaction rate of 6.5% per min in the earlystage of reaction and where it returns to that value in the laterstages. Low values of T_(critical) and T_(late) indicate a reactivematerial, and vice versa. Some selected results for these indices areincluded in Table 2 (which follows) for the composite mixture chars.These values are comparable and indicate that these chars are veryreactive and would be easy to gasify or combust in order to recoveradditional carbon and nitrogen. The same conclusions were reached inmore extensive char characterization studies previously carried out,which also included characterization of pore structure.

From the initial pore structure characterization work performed on thecomposite waste chars, it appears that they have porositycharacteristics similar to those encountered with pyrolysis of woods.Surface areas and accessible porosity are both quite low. It is expectedthat extending measurements to samples of waste-derived char materialsthat have been oxidized to a significant degree will establish whetherthe waste-derived chars develop significant porosity, just as do thewood chars. This can have important consequences not only for thereactivity of the char in gasification, but also in the possible furtheruseful application of these materials (e.g., as adsorbents) in a spacecabin environment.

TG-FTIR Experiments with the Post Pyrolyzer (TG-FTIR/PP)

The TG-FTIR system was used as discussed above to characterize theprimary pyrolysis behavior of the individual components and thecomposite sample. In this phase, the system was also equipped with apost-pyrolysis system (isothermal secondary pyrolysis unit) in order tostudy the cracking of the heavy liquids (tars) and other volatiles thatare formed during pyrolysis of these materials. This post-pyrolysis unitcan be operated from 500-1000° C. with an average volatile residencetime of 0.4-2.6 seconds at atmospheric pressure. Under the rightpyrolysis conditions, the liquids are cracked to produce primarily CO,CO₂, CH₄, H₂, H₂O, and small amounts of carbon.

The experiments were done with the TG-FTIR/PP system over thetemperature range from 600-1000° C. in the post pyrolyzer. The heliumgas flow rate through the 14 cm³ volume post pyrolyzer for the “fast”runs was ˜400 cm³/min (at standard conditions). Additional runs weredone at lower flow rates (˜100 cm³/min) in order to test the effect ofthis variable and also to provide gas concentration levels that wouldallow for simultaneous measurements by FT-IR and GC. Over a temperaturerange of 600-1000° C., these gas flow rates correspond to a range ofresidence times for the fast flow conditions of 0.4 to 0.6 second and1.8 to 2.6 seconds for the slower flow conditions; i.e., the flow rateswere not adjusted to equalize the residence times at each temperature.

The TG-FTIR/PP experiments were done for both the composite mixturesample and also the wheat straw sample. A set of results for thecomposite mixture, shown in Table 2, demonstrate the very strong effectof the post pyrolysis temperature on the product composition. As thepost pyrolysis temperature increases, the tar yields decline to zero andthe CO yields increase dramatically. The CH₄, H₂O and CO₂ yields gothrough a maximum. Similar results are observed for post-pyrolysis runsdone with the pure wheat straw sample. In order to get yield data on H₂,the GC (gas chromatograph) system was used to take periodic samples.

TABLE 2 Results of TG-FTIR/Post Pyrolyzer Experiments for the CompositeMixture Temperature ° C. Temp. 600° C. 700° C. 800° C. 900° C. 1000° C.Flow Rate Slow Fast Slow Fast Slow Fast Slow Fast Slow Fast Moisture 3.13.9 3.9 4.5 9.1 6.8 9.9 4.9 Volatiles 87.0 81.9 56.0 80.8 75.2 75.5 74.777.8 Fixed Carbon 11.9 12.8 9.2 11.3 12.8 12.2 11.9 11.3 Ash 2.0 1.430.9 3.4 3.0 5.5 3.5 6.0 Tars 30.2 0.5 0.0 0.0 1.1 0.0 1.1 0.0 CH₄ 1.633.85 7.69 6.56 7.97 5.0 6.44 4.10 H₂O (pyr) 18.32 17.34 11.11 18.5211.02 13.43 8.02 12.50 H₂ 1.92 4.61 7.54 CO₂ 10.60 13.40 17.77 15.0722.40 13.16 21.60 15.80 CO 6.14 23.51 15.66 22.62 30.20 23.80 45.5036.15 NH₃ 0.85 0.88 0.36 0.68 0.38 0.48 0.39 0.22 C₂H₄ 4.45 16.26 6.9511.08 1.88 5.05 0.92 2.06 COS 0.35 0.54 0.46 0.49 0.45 0.61 0.60 0.46SO₂ 0.91 0.80 0.00 0.70 0.43 0.94 0.84 0.87 CHNO 1.40 2.12 0.06 0.090.01 0.26 0.02 0.34 C₃H₃N₃O₃ 0.01 0.00 0.00 0.00 0.01 0.00 0.00 0.00C₄H₁₁NS 5.28 4.49 0.00 0.00 0.00 0.00 0.00 0.00 T_(critical) 346 374 362384 414 381 360 378 T_(late) 512 493 470 500 444 500 398 498 Notes:Yields are given on an as-received wt. % basis; in cases where twoexperiments are done, the results are averaged; H₂ yields are from GCmeasurements; fast flow rate was ~400 cm³/min at standard conditions(0.4-0.6 second residence times), while the slow flow rate was ~100cm³/min (1.8-2.6 second residence times); T_(critical) and T_(late) areindices of the char reactivity (see above).

The H₂ measurements were made for selected experiments. Since the GC wasnot used to monitor the entire evolution profile, the complete H₂ yieldwas calculated by using the CH₄ and CO yields as internal standards,since these gases are measured both by FT-IR and GC. In order toestimate the H₂ yields for experiments where no GC measurements aremade, a correlation was made between the existing H₂ yield data and theCO yields. Although this correlation consisted of only three points, itwas linear over a wide range of CO and H₂ concentrations and was used tointerpolate the results for the remainder of the experiments in Table 2.Based on these estimates, the change in the molar gas composition forthe fast flow experiments with the composite mixture was determined, asshown in Table 3 (excluding the composition of the inert helium carriergas). The data in Table 3 show that with increasing pyrolysistemperature, the gas composition becomes rich in H₂ and CO and that CH₄,CO₂ and H₂O are also key components. While tars and minor heteroatomicspecies are present at low temperatures, these are largely eliminated asthe temperature increases.

Summary of Effects of Pyrolysis Conditions on Yields

Changes in the char yields were observed in the DSC experiments for thecellulose, polyethylene, and the composite mixture when the degree ofconfinement was changed in the sample holder, as discussed above. Theseresults, along with the post pyrolysis results in Tables 2 and 3,underscore the significant effect of primary and secondary pyrolysisconditions on the final product mix. There are many variables that canbe manipulated for pyrolysis that can be used to compensate for changesin the feedstock composition and/or the desired product yields (e.g.,time-temperature history, pressure). This provides a much greater degreeof control over the solid waste processing step than is possible foreither gasification or incineration. Changing the pyrolysis conditionsallows one to effect significant changes in the pyrolysis productdistribution (char, tar, gas) and the gas composition. Liquids can beproduced if desired (under mild conditions) or cracked to form carbonoxides and fuel gases under severe conditions, depending on what isrequired for the life support system.

TABLE 3 Estimated Gas Phase Composition (Mole %) from TG-FTIR/PostPyrolyzer Experiments with the Composite Mixture Temperature ° C. 600°C. 700° C. 800° C. 900° C. 1000° C. CH₄ 4 5 9 7 5 H₂O 44 20 21 18 12 H₂^(a) 11 36 36 42 51 CO₂ 11 6 7 7 7 CO 10 17 17 20 24 C₂H₄ 7 12 8 4 1 NH₃2 1 1 1 <<1 COS <<1 <<1 <<1 <<1 <<1 SO₂ <1 <<1 <<1 <<1 <<1 CHNO 1 1 0<<1 <<1 C₃H₃N₃O3 0 0 0 0 0 C₄H₁₁NS 2 1 0 0 0 Tars 7 0 0 0 0 Notes: Datafrom fast flow condition (400 cm³/min) ^(a)estimated from correlationbetween H₂ and COModeling of Primary Pyrolysis

Most of the composite mixture consists of materials which are polymericin nature (polyethylene, cellulose, wheat straw). Consequently, the useof modeling approaches that have previously been successful forpolymeric materials is appropriate.

Statistical network models—The important processes in the early stagesof pyrolysis of polymeric materials are polymerization/depolymerization,cross-linking and gas formation, and it is known that these earlyprocesses determine the composition of the products. The geometricalstructure of a polymer (whether it is chain like or highly cross-linked)controls how it reacts under otherwise identical chemical reactions.One, therefore, can often use statistical models based on thegeometrical structure to predict the reactions of a polymer. Suchstatistical models have been developed, for example, to describe thethermal decomposition of coal, lignin, and phenol-formaldehyde.

The general model developed to describe thermal decomposition ofcrosslinked aromatic polymer networks is called the FG-DVC model; it isdescribed in the prior art. In developing the model, extensiveexperimental work was done with synthetic polymers to allow the study ofbond breaking and mass transport in chemically clean systems. The modelcombines two previously developed models, a Functional Group (FG) modeland a Depolymerization-Vaporization-Crosslinking (DVC) model. The DVCsubroutine is employed to determine the amount and molecular weight ofmacromolecular fragments. The lightest of these fragments evolves astar. The FG subroutine is used to describe the gas evolution and theelemental and Functional Group compositions of the tar and char. In thecase of coal or lignin, cross-linking in the DVC subroutine is computedby assuming that this event is correlated with CO₂, CH₄, and/or H₂Oevolutions predicted in the FG subroutine.

Model parameters—The implementation of the FG-DVC model for a complexpolymeric material requires the specification of several parameters,some of which can be constrained by the known structural units and someof which are constrained by experimental characterization data. Thebasic idea is to calibrate the model using simple small scale pyrolysisexperiments like TG-FTIR and pyrolysis-Field Ionization MassSpectrometry (FIMS), and then use the model to make predictions forconditions where experimental data are not readily available.

Modeling of whole biomass—In more recent work, sponsored by the USDA,the FG-DVC model was applied to the pyrolysis of plant biomass samples.A number of biomass samples were considered based on the abundance andavailability of agricultural and forestry feedstocks and waste materialsin the United States. Six samples were obtained from the NationalInstitute of Standards and Technology's Standard Reference MaterialsProgram which included microcrystalline cellulose (C), sugar canebagasse (B), wheat straw (WS), corn stalk (CS), softwood Pinus radiata(PR), and hardwood Populous deltoides (PD). A biomass classificationscheme was developed based on comparing the placement of a nearreference set of samples on a van Krevelen diagram (plot of H/C vs. O/Catomic ratio).

In the case of whole biomass, the pyrolysis behavior is dominated by thecellulose and hemi-cellulose, non-aromatic, components. Consequently,the DVC model, as currently formulated, is not as useful as in the caseof aromatic polymers like coal or lignin. For these types of materialsand for mixed waste streams, such as would be found onboard aspacecraft, the FG-DVC model is still used, but the DVC portion has beenlargely disabled. This means that the ultimate liquid (tar) yield is anadjustable parameter.

The FG-DVC model predictions were compared with the yields andcomposition of pyrolysis products from the TG-FTIR experiments for wholebiomass samples. The model was subsequently used to make predictions ofpyrolysis product distributions over a wider range of conditions andcompares reasonably well to biomass pyrolysis data obtained at muchhigher heating rates.

Modeling of pyrolysis of mixed waste streams—The detailed modeling ofthe pyrolysis of mixed waste streams has not been extensively studied,except for mixtures of whole biomass components (cellulose,hemicellulose, lignin), municipal solid waste, and scrap tire components(carbon black, extender oil, natural rubber, butadiene rubber, andstyrene-butadiene rubber). Additive models based on a linearcontribution of the component species work reasonably well for overallweight loss but less well for individual pyrolysis yields, especially inthe case of biomass. In the present instance, the first task in themodeling work was to use the FG-DVC pyrolysis model to simulate theprimary pyrolysis data from the TG-FTIR experiments for the majorconstituents. The TG-FTIR data are used as generated (see FIGS. 1 and2), except for the tar evolution rate which is adjusted to reflect anylack of mass balance closure in the original experiment.

In general, the yields of individual products and the overall weightloss were well predicted for wheat straw. The next step was to use theFG-DVC model to simulate the primary pyrolysis data from TG-FTIRexperiments with the composite mixture. This was done initially byassuming that the same input parameters as for wheat straw would applyand making a modification to the tar evolution pool to allow for twoevolution peaks from the composite mixture (only one tar peak isobserved from wheat straw pyrolysis). The predictions and experimentaldata are compared in FIG. 3 at a heating rate of 30° C./min. Goodagreement between the predictions and the experimental data wasobserved, except for small evolution peaks for CO₂, H₂O and CH₄. Theseextra peaks can be easily modeled by adjusting the distribution of gaspools in the model input file. In addition, the model has recently beenimproved so that equally good fits were obtained using the actualelemental composition for the composite mixture.

Modeling of Primary and Secondary Pyrolysis Behavior

In order to develop the complicated relationship between the compositionof the starting materials, the process conditions and the desiredproduct yields, this work has also investigated the use of artificialneural network (ANN) models. Recently, ANNs have been applied to avariety of similarly intractable problems and have demonstrated a highdegree of success. The ability of ANNs to learn from observation,together with their inherent ability to model nonlinearity, make themideally suited to the problem of control in complex pyrolysis processes.It should be possible to use ANNs to adaptively model the pyrolysisprocess using the process parameters as inputs and the resultingpyrolysis product distributions as outputs. The model would then be usedin a feedback control loop to maximize the yields of desirable productswhile minimizing side reactions. The validation data for the ANN controltechnology will be the concentrations of pyrolysis species supplied byIR gas analysis equipment.

A basic neural network development software package has been developedfor National Instrument's (Austin, Tex.) LabVIEW software. Working inthe LabVIEW environment provides for a flexible user interface, and easyaccess to many types of data. The Neural Network Development for LabVIEW(NNDLab) software includes tools to extract custom data sets directlyfrom VISTA software. Backpropagation networks can be trained and testedusing delta rule, delta-bar-delta rule, or extended delta-bar-delta ruleparadigms, and could easily be imbedded into dedicated analysis orprocess control LabVIEW programs. This package was used to controlNO_(x) in a selective non-catalytic reduction (SNCR) process developedat Nalco Fuel Tech. (NFT). A data set was collected using in-situmeasurements of NO, CO, and NH₃ by FT-IR and for six process setpointsand the transition periods between the setpoints.

A typical ANN is made up of three layers of processing units (nodes) andweighted connections between the layers of nodes. The input data areintroduced at the input layer and are fed to the hidden layer throughthe weighted connections. As discussed by Psichogios and Ungar, ANNshave typically been used as “black-box” tools; i.e., assuming no priorknowledge of the process being modeled. One variation of this approachis to create a hybrid ANN which combines a known first principles modelwith a neural network model. This makes the ANN more robust and easierto train. An approach for future examination is to use the FG-DVC modelfor the mixed waste stream as the first principles model and develop ahybrid ANN model to describe the pyrolysis reactor.

ANN modeling results—A prerequisite to testing of the concept of usingan ANN as the basis for a control scheme is that an ANN must first beable to model the important inputs and outputs in a system. In order totest this idea, yields of CO for the composite sample from thepost-pyrolyzer experiments were calculated using a back-propagationneural network model. Five sets of test data (at 600, 700, 800, 900,1000° C.) were divided into training and test sets for the neuralnetwork model. The results of model predictions for CO based on atraining set using the data in Table 2 are shown in FIG. 4. FIG. 5compares the predicted and measured product yields for all of the “fastflow” experiments with the composite mixture. In this case, the modelwas trained on the average results and then used to predict theindividual results. The fact that the gas yields can be well correlatedusing ANN methods implies that monitoring 3-4 gases will be adequate tocontrol the process and that the inability of IR methods to measurehydrogen will not be a problem.

FIG. 6 diagrammatically illustrates a system embodying the presentinvention. The reactor, generally designated by the numeral 10, consistsof a first stage, comprised of chamber 12, for effecting pyrolysis,operatively connected to a second stage, comprised of chamber 14 andcontaining a bed 16 of silica gel-based catalyst. Fuel gas withdrawnfrom chamber 14 is passed through a flow-control unit 18 (which may be avalve, a holding tank, etc.), and thereafter passes to a power generator20 (e.g., a fuel cell, an internal combustion engine, a Stirling engine,or a thermophotovoltaic (TPV) system). Flow rates of gases (e.g., O₂,CO₂ and H₂O) into the reactor 10 are controlled by a bank 22 of flowcontrollers, and overall control of those flow rates and of the flowrate of gases out of the reactor 10, as well as of temperatures andother process conditions within the reactor, are monitored andcontrolled by an electronic data processing unit (EDP) 24, programmed asherein set forth.

Conclusions

The feasibility of pyrolyzing a representative composite mixture ofmixed solid waste materials, and producing usable gases (CH₄, H₂, CO₂,CO, H₂O, NH₃) as the main products, and a reactive carbon-rich char asthe main byproduct, have been demonstrated. Significant amounts ofliquid products are formed under less severe pyrolysis conditions, butthese were cracked almost completely to gases as the secondary pyrolysistemperature was raised. A primary pyrolysis model was developed for thecomposite mixture based on an existing model an for whole biomassmaterials, and an ANN model was used successfully to model the changesin gas composition with pyrolysis conditions. It is demonstrated thatpyrolysis processing meets the requirements of solid waste resourcerecovery in space; i.e., it produces usable byproducts, with minimalside products which can be tailored to meet changes in the feedstockcomposition and the product requirements, significantly reduces storagevolume, requires low maintenance, can be conducted as a batch,low-pressure process, and is compatible with the utilities that arepresent on board a spacecraft (electricity and small amounts of O₂ andH₂O). Although the pyrolysis gases may require further treatment, suchas water gas shift conversion to remove CO, before they can be ventedinto the cabin, these gases could be introduced into an incinerator or ahigh temperature fuel cell system with minimal pretreatment.

A prototype waste pyrolysis system, related to the present invention,would be useful to NASA in at least four respects: 1) it can be used asa pretreatment for an incineration process; 2) it can be used as a moreefficient means of utilizing oxygen and recycling carbon and nitrogen;3) it can be used to supply fuel gases to fuel cells for powergeneration; and 4) it can be used as the basis for the production ofchemicals and materials in space.

The invention addresses an important problem for long term space travelactivities; i.e., closed loop regenerative life support systems. Whilethe problem of solid waste resource recovery has been studied for manyyears, there is currently no satisfactory waste disposal/recyclingtechnology. Pyrolysis processing is a very versatile technology, asdiscussed above, and can accommodate long term needs for a CELSS. Unlikeincineration, the issue of CO₂ management can be largely decoupled fromthe issue of waste management.

Thus, it can be seen that the present invention provides a novel processand system by which non-gaseous hydrocarbonaceous materials, andparticularly mixed solid waste materials, can be converted to usablegases as the main products, and to a reactive carbon-rich char as themain byproduct. The process is feasible for use in a controlledecological life support system, and enables the provision of such asystem.

1. A process for producing fuel gases from at least one non-gaseoushydrocarbonaceous material, using a two-stage reaction apparatus,comprising the following steps, at least steps (b), (c), (d), and (e)being carried out cyclically: (a) introducing a non-gaseoushydrocarbonaceous material into a pyrolysis chamber, comprising a firststage of the apparatus; (b) pyrolyzing said hydrocarbonaceous materialin said first stage pyrolysis chamber so as to produce a primary fuelgas mixture, a pyrolysis liquid, and a first carbonaceous residue; (c)introducing said primary fuel gas mixture and pyrolysis liquid into asecond chamber, comprising a second stage of the apparatus andcontaining a catalyst, and heating said liquid therein, in asubstantially non-oxidizing atmosphere, to a temperature of 900° to1100° C. and substantially above the temperature at which pyrolysis iseffected in step (b), so as to produce additional fuel gases andadditional solid carbonaceous residue, without substantially alteringthe composition of said primary fuel gas mixture; (d) withdrawing saidprimary fuel gas mixture and said additional fuel gas from said secondchamber; and (e) introducing air, oxygen, carbon dioxide or steam intoeach of said chambers to effect reaction with, and at least partialremoval of, said carbonaceous residue therein.
 2. The process of claim 1wherein said carbonaceous material is heated to a temperature of about500° to 600° C. in said pyrolysis step (b), wherein said primary fuelgas mixture consists primarily of carbon monoxide, methane, andhydrogen, and wherein said catalyst in said second chamber is asilica-gel based catalyst.
 3. The process of claim 1 wherein steam,carbon dioxide, or a mixture thereof is introduced into each of saidchambers in said step (e) so as to produce a further quantity of fuelgas.
 4. The process of claim 1 wherein said introducing step (e) effectsregeneration of said catalyst in said second chamber.
 5. The process ofclaim 1 wherein said steps (b), (c), (d) and (e) are controlled byelectronic data processing means programmed to monitor the formation ofat least one gas phase product.
 6. The process of claim 5 wherein saidat least one gas phase product is selected from the group consisting ofhydrogen, methane, carbon monoxide, carbon dioxide, water, and oxygen.7. The process of claim 5 wherein the formation of at a least three gasphase products are monitored for controlling said steps.
 8. The processof claim 7 wherein said data processing means is programmed to determinethe concentrations of said at least three gas phase products.
 9. Theprocess of claim 8 wherein said data processing means is programmed toproduce a fuel gas product of selected composition, from a specifiedhydrocarbonaceous material, by controlling the operating parameters forsaid first and second stages of the said apparatus.
 10. The process ofclaim 1 wherein said process is a batch process.
 11. The process ofclaim 2 wherein said catalyst in said second chamber is a fixed bed ofsilica gel-based catalyst.
 12. The process of claim 8 wherein said dataprocessing means is additionally programmed to implement an artificialneural network model based upon the concentrations of said at leastthree gas phase products, said concentrations being utilized as inputdata to said neural network.
 13. A batch process for producing fuelgases from at least one non-gaseous hydrocarbonaceous material, using atwo-stage reaction apparatus, comprising the following steps, at leaststeps (b), (c), (d) and (e) being carried out cyclically: (a)introducing a non-gaseous hydrocarbonaceous material into a pyrolysischamber, comprising a first stage of the apparatus: (b) pyrolyzing saidhydrocarbonaceous material in said first stage pyrolysis chamber so asto produce a primary fuel gas mixture, a pyrolysis liquid, and a firstcarbonaceous residue; (c) introducing said primary fuel gas mixture andpyrolysis liquid into a second chamber, comprising a second stage of theapparatus and containing a non-consumable catalyst, and heating saidliquid therein, in a substantially non-oxidizing atmosphere, to atemperature of 900° to 1100° C. and substantially above the temperatureat which pyrolysis is effected in step (b), so as to produce additionalfuel gases and additional solid carbonaceous residue, withoutsubstantially altering the composition of said primary fuel gas mixture;(d) withdrawing said primary fuel gas mixture and said additional fuelgas from said second chamber; and (e) introducing air, oxygen, carbondioxide or steam into each of said chambers to effect reaction with, andat least partial removal of, said carbonaceous residue in said chambers,said steps (b), (c), (d) and (e) being controlled by electronic dataprocessing means programmed to monitor the formation of at least one gasphase product.
 14. The process of claim 13 wherein said at least one gasphase product is selected from the group consisting of hydrogen,methane, carbon monoxide, carbon dioxide, water, and oxygen.
 15. Theprocess of claim 13 wherein the formation of at least three gas phaseproducts are monitored for controlling said steps (b), (c), (d) and (e).16. The process of claim 15 wherein said data processing means isprogrammed to determine the concentrations of said at least three gasphase products.
 17. The process of claim 13 wherein said data processingmeans is programmed to produce a fuel gas product of selectedcomposition, from a specified hydrocarbonaceous material, by controllingthe operating parameters for said first and second stages of the saidapparatus, and to effect said process in closed-loop mode.
 18. Theprocess of claim 13 wherein said catalyst in said second chamber is afixed bed of silica gel-based catalyst.